Functional fluid compositions

ABSTRACT

PRODUCTION OF FUNCTIONAL FLUIDS FOR AIRCRAFT, INDUSTRIAL AND MARINE USE, HAVING GOOD FIRE RESISTANCE, DESIRABLE VISCOSITY CHARACTERISTICS, LOW DENSITY AND WHICH ARE RELATIVELY INEXPENSIVE, ESPECIALLY USEFUL AS AIRCRAFT HYDRAULIC FLUIDS, COMPRISING A MIXTURE OF A PHOSPHORUS ESTER, SUCH AS A PHOSPHATE, E.G., TRIBUTYL PHOSPHATE, AND AN ALKYL DIESTER OF PHTHALIC ACID, SUCH AS DIISODECYL PHTHALATE, AND WHICH MAY ALSO CONTAIN AN ALKYL DIESTER OF ADIPIC OR SEBACIC ACID, SUCH AS DIISODECYL ADIPATE.

3,795,620 FUNCTIONAL FLUID COMPOSITIONS Martin B. Sheratte, Reseda, Calif., assignor to McDonnell Douglas Corporation, Santa Monica, Calif. No Drawing. Filed Mar. 29, 1971, Ser. No. 129,270

Int. Cl. C09k 3/00 US. Cl. 252-78 12 Claims ABSTRACT OF THE DISCLOSURE This invention relates to functional fluid compositions having good fire resistance and desirable viscosity characteristics at both high and low temperatures, and is particularly directed to functional fluid compositions having the above noted properties and consisting essentially of a phorphorus ester, and one or more of certain dicarboxylic .acid diesters, particularly a phthalate diester, the resulting functional fluids being relatively inexpensive while retaining the desirable characteristics and advantages of conventional functional fluids containing chiefly a phosphate ester or mixtures thereof.

Many different types of materials are employed as functional fluids, and functional fluids are utilized in a wide variety of applications. Thus, such fluids have been utilized as electronic coolants, diffusion pump fluids, lubricants, damping fluid, power transmission and hydraulic fluids, heat transfer fluids and heat pump fluids. A particularly important application of such functional fluids has been their utilization as hydraulic fluids and lubricants in aircraft, requiring successful operation of such fluids over a wide temperature range, and fire resistant fluids.

Functional and hydraulic fluids employed in many industrial applications and particularly hydraulic fluids for aircraft must meet a number of important requirements. Thus, such hydraulic fluids particularly for aircraft use, should be operable over a wide temperature range,

should have good stability at relatively high temperatures and preferably have lubricating characteristics. In addition to having the usual combination of properties making it a good lubricant or hydraulic fluid, such fluid should also have relatively low viscosity at extremely low temperatures and an adequately high viscosity at relatively high temperatures, and must have adequate stability at the high operating temperatures of use. Further, it is of importance that such fluids be compatible with and not adversely affect materials including metals and non-metals such as seals, of the system in which the fluid is employed. It is also important in aircraft hydraulic fluids and lubricants that such fluids have as high a fire resistance as possible to prevent ignition if such fluids are accidentally or as result of damage to the hydraulic system, sprayed onto or into contact with surfaces or materials of high temperature. Another important property particularly for application of a hydraulic fluid in aircraft is the provision of a low density fluid to increase pay load.

United States Patent 3,795,620 Patented Mar. 5, 1974 ICC Hydraulic fluids in commercial jet aircraft are exposed to temperatures ranging from below 40 F. to over 200 F. Within these temperature extremes, it is necessary for the fluid to maintain a reasonably low viscosity when cold, and yet not become too thin when hot. As a general rule, this means that the fluid should have a viscosity of less than 1,000 cs. (centistokes), and preferably less than 500 cs., at 40 F., and maintain a viscosity above 2.0 cs. at 210 F.

Presently available commercial functional or hydraulic fluid base stocks do not possess these viscosity characteristics. Phosphate esters are among the most commonly employed base stocks, of which tributyl phosphate and dibutyl phenyl phosphate are 'widely used components. Both of the latter phosphates are too thin at high temperatures, and their use alone would result in rapid wear of moving parts. Other phosphate esters, such as tricresyl phosphate, for example, which provide the requisite high temperature viscosity become too thick to be useful at low temperatures. Even mixtures of various phosphate esters such as those noted above do not provide the required viscosity characteristics at both low and high temperatures. Accordingly, it has been the practice to achieve the required wide viscosity range required for aircraft hydraulic fluids by adding to a thin base stock, such as phosphate ester or mixtures thereof, a small proportion, e.g., up to 10%, of a polymeric material such as polyalkyl acrylates or methacrylates, whose solubility characteristics in the base stock are chosen so that the polymeric material thickens the fluid more at high temperatures than at low temperatures, and thus functions as a viscosity index (VI) improver.

However, the chief disadvantage of this method of viscosity improvement is that polymers of the type noted above are generally not stable to the shearing forces encountered in an aircraft hydraulic system, and during use, these large molecules are sheared down to smaller molecules which lose their ability to improve viscosity index. This means that the functional fluid loses viscosity in use and requires addition of polymer to the fluid and eventually, as the detritus from polymeric breakdown accumulates in the fluid and contaminates it, the fluid must be discarded. In addition, of course, employment of a viscosity index improver in the functional fluid, even in the minor amounts presently employed, substantially increases the cost of the fluid.

It is accordingly an object of this invention to provide a functional fluid which is particularly useful as an air craft hydraulic fluid and which has requisite viscosity characteristics over a wide temperature range, and can be employed with or without viscosity index improvers, while at the same time having the other requisite properties for a good hydraulic fluid, including good fire resistance and low density, and of substantially reduced cost as compared to presently available aircraft hydraulic fluids, particularly those based essentially on phosphorus esters as the base stock components. Hence it is a particular object to replace such presently employed relatively expensive phosphorus ester base stock fluids, With functional fluids or blends containing base stock components in addition to phosphorus ester or phosphate components, which confer on the functional or hydraulic fluid good viscosity and density characteristics, While substantially reducing the cost of the fluid.

The above objects are achieved according to the present invention by the provision of a functional fluid, particularly valuable as a hydraulic fluid for aircraft, comprising basically a mixture of a phosphorus ester and a phthalic acid diester. Such mixture can also contain optionally, but not necessarily, a diester of an aliphatic dicarboxylic acid selected from the group consisting of adipic and sebacic acids, chiefly to provide greater versatility with respect to tailoring the functional fluid to obtain a desirable low viscosity at low temperatures below -40 F. and to increase the viscosity at high temperatures greater than 210 F. The phosphorus ester and diester components, particularly the diester of phthalic acid, are employed preferably in the amounts or proportions set forth hereinafter, in order to obtain the desirable properties of the functional fluid noted above.

Of particular significance, the functional fluids produced according to the invention have a fire resistance comparable to the dire resistance of presently employed commercially available hydraulic fluids, and can be blended to have suitably low viscosity at temperatures below --40 F. and suitably high viscosity at high temperatures above 210 F., and this can be accomplished with or Without employment of a viscosity index irnprover. By avoiding the necessity for a viscosity index improver, the functional fluids of the invention do not suffer from the disadvantage noted above, namely the deterioration of such polymeric additives used for viscosity improvement, and accumulation of molecular debris, leading to a shortening of the useful life of the fluid. Hence the functional fluids of the present invention have a longer period of usefulness, providing economic advantages including the avoidance of the high cost of the viscosity index improver and the employment of relatively low cost diesters such as the above noted phthalates, and also the above noted adipates and subacates where employed, in place of a substantial portion of the phosphate ester generally employed in presently available aircraft hydraulic fluids.

Also, the functional fluid base stocks of the present invention have good thermal and hydrolytic stability comparable 'with the phosphate ester based fluids currently in use, contributing to a long useful life for the fluid. In addition, the functional fluids according to the present invention have low densities of the order of 1.0 or less, an important property for aircraft hydraulic fluids. The above advntages have been achieved without any sacrifice in the desirable flammability characteristics which are exhibited by the phosphate ester type hydraulic fluids currently in use. Also, the functional fluids according to the invention have freedom from corrosivity and wear with respect to the metallic and non-metallic components, and the pumps of hydraulic fluid systems, which compare favorably with these characteristics for phosphate type base stock hydraulic fluids presently commercially employed.

Further, in addition to their valuable application as hydraulic fluids for aircraft hydraulic systems, the fluids according to the invention have important application as a hydraulic or functional fluid in industrial and marine fields, particularly in industrial turbine systems.

One of the essential components of the functional fluids according to the invention is a phosphorous ester having the general formula:

Rr-On where s, m and n can be or 1, and not more than two of s, m and n can be 0, where R R and R each can be aryl such as phenyl and naphthyl, alkaryl such as cresyl, xylyl, ethyl phenyl, propyl phenyl, isopropyl phenyl, and the like, said aryl and alkaryl radicals preferably containing from 6 to about 8 carbon atoms, alkyl, both straight chain and branched chain of from about 3 to about 10 carbon atoms such as n-propyl, n-butyl, n-amyl,

n-hexyl, isopropyl, isobutyl, isoamyl, and the like, and alkoxyalkyl having from about 3 to about 8 carbon atoms such as methoxy methyl, methoxy ethyl, ethoxy ethyl, methoxy propyl, and the like.

The corresponding phosphonates can also be employed, where one of s, m and n is 0, and the corresponding phosphinates where two of s, m and n are 0.

The phosphate esters, that is, where s, m and n are each 1, are the preferred phosphorus esters, and the preferred phosphate esters are the dialkyl aryl, triaryl, trialkyl, and alkyl diaryl phosphates.

Examples of such phosphate esters are the dialkyl aryl phosphates in which the alkyl groups are either straight chain or branched chain and contain from about 3 to about 10 carbon taoms, such as n-propyl, n-butyl, n-amyl, n-hexyl, isopropyl, isobutyl, isoamyl, and the aryl radicals have from 6 to 8 carbon atoms and can be phenyl, cresyl or xylyl, particularly dialkyl phenyl phosphates including dibutyl phenyl phosphate, butyl amyl phenyl phosphate, butyl hexyl phenyl phosphate, butyl heptyl phenyl phosphate, butyl octyl phenyl phosphate, diamyl phenyl phosphate, amyl hexyl phenyl phosphate, amyl heptyl phenyl phosphate, and dihexyl phenyl phosphate.

Examples of triaryl phosphates are those in which the aryl radicals of such phosphates have from 6 to 8 carbon atoms, that is, may be phenyl, cresyl or xylyl, and in which the total number of carbon atoms in all three of the aryl radicals is from 19 to 24, that is, in which the three radicals include at least one cresyl or xylyl radical. Examples of such phosphates include tricresyl, trixylyl, phenyl dicresyl, and cresyl diphenyl phosphates.

Examples of trialkyl phosphates employed according to the invention include phosphates having alkyl groups which are either straight chain or branched chain with from about 3 to about 10 carbon atoms, such as n-propyl, n-butyl, n-amyl and n-hexyl, particularly tri-n-butyl phosphate, tri(2-ethyl hexyl) phosphate and triisononyl phosphate, the straight chain alkyl groups preferably containing from 4 to 6 carbon atoms.

Examples of alkyl diaryl phosphates which can be employed to produce the invention compositions include those in which the aryl radicals of such phosphates may have from 6 to 8 carbon atoms and may be phenyl, cresyl or xylyl, and the alkyl radical may have from about 3 to about 10 carbon atoms, examples of which are given above. Examples of the alkyl diaryl phosphates include butyl diphenyl, amyl diphenyl, hexyl diphenyl, heptyl diphenyl, octyl diphenyl, G-methyl heptyl diphenyl, butyl phenyl cresyl, amyl phenyl xylyl, and butyl dicresyl phosphates.

Particularly desirable phosphate esters are tributyl phosphate, dibutyl phenyl phosphate and tricresyl phosphate, and mixtures thereof.

Functional fluid base stocks according to the invention also include phosphonate and phosphinate esters having alkyl and aryl groups corresponding to those defined above with respect to the phosphate esters.

Examples of phosphinate esters to which the invention principles are applicable include phenyl-di-n-propyl phosphinate, phenyl-di-n-butyl phosphinate, phenyl-di-n-pentyl phosphinate, p methoxyphenyl-di-n-butyl phosphinate, tert-butylphenyl-di-n-butyl phosphinate. Examples of phosphonate esters to which the invention is applicable include aliphatic phosphonates such as an alkyl alkenyl phosphonate, e.g., dioctyl isooctene phosphonate, an alkyl alkane phosphonate such as di-n-butyl n-octane phosphonate, di-isooctyl pentane phosphonate, and dimethyl decane phosphonate, a mixed alkyl aryl phosphonate, for example, di-octyl phenyl phosphonate, di(n-amyl) phenyl phosphonate, di(n-butyl) phenyl phosphonate, phenyl butyl hexane phosphonate and butyl bis-benzene phosphonate.

Although a single phosphorus ester may be employed, in many instances it is desirable to employ a mixture of phosphorus esters in order to obtain the desired propert ties, e.g., viscosity as well as fire resistance, such as mixtures of phosphates, e.g., a mixture of tributyl and tricresyl phosphates, mixtures of phosphates and phosphonates, mixtures of phosphates and phosphinates, and the like.

The diethers of phthalic acid, the second essential component of the functional fluid compositions of the invention, are aliphatic or alkyl diesters, specificallydialkyl phthalates, containing alkyl groups, either straight chain or branched chain, of from about 4 to about 12 carbon atoms, examples of which include butyl, isbutyl, amyl, pentyl, hexyl, isohexyl, nonyl, decyl, and isodecyl groups. Specific illustrative examples of the diester phthalates which can beemplo'yed are di-n-butyl phthalate, dihexyl phthalate, dioctyl phthalate, dinonyl phthalate, didecyl phthalate, and diisodecyl phthalate.

The optional components which can, but need not necessarily, be employed in the functional fluid compositions of the invention, are the aliphatic or alkyl diesters of adipic and sebacic acid, that is the dialkyl adipates and the dialkyl sebacates, which contain alkyl groups, either straight chain or branched chain, containing from about 4 to about 12 carbon atoms, examples of which alkyl groups are given above with respect to the diesters of phthalic acid which can be employed. Specific illustrative examples of these adipate and sebacate diesters are dihexyl, di(2-ethylhexyl), dioct'yl, dinonyl, didecyl and diisodecyl adipates, and the corresponding sebacates. If desired, a. mixture of adipate esters, a mixture of sebacate esters or a mixture of adipate and sebacate esters can be employed.

Generally, a substantial proportion of both the phosphorus ester, particularly phosphate ester, and diester, particularly phthalate diester, are employed in the invention composition. Thus, the amount of phosphorus ester, e.g., phosphate, present in the invention composition can range, by weight, from about 15 to about 75%, preferably about 20 to about 60%, the amount of phthalate diester can range from about 5 to about 75 preferably about to about 50%, and the amount of adipate and/or sebacate diester employed can range from 0 to about 50% preferably from about 10 to about 40%, where employed. Where only a phosphorus, e.g., phosphate,-ester and a phthalate diester are employed as the essential base stock components according to the invention, it is preferred to employ such components in proportions ranging, by weight, from about 40 to about 75% of the phosphorus, e.g., phosphate, ester and from about to about 60% phthalic acid or phthalate diester.

As previously noted, a feature of the present invention is that in many instances suitable viscosity characteristics both at low temperatures below --40 F. and at high temperatures above 210 F. can be achieved in the absence of a viscosity index improver. However, it may be desirable, particularly for improving pump life in a hydraulic system, to incorporate a small amount ranging from 0 to about 10%, generally about 2 to about 10%, by weight of the composition, of a viscosity index improved. Examples of the latter are polyalkyl acrylates and polyalkyl methacrylates, in which the alkyl groups may contain from about 4 to about 12 carbon atoms, either straight or branched chain, and having an average molecular weight ranging from about 6,000 to about 15,000. Specific examples of such viscosity index improvemers are polybutyl methacryl-ate and poly n-hexyl acrylate, having an average molecular weight ranging between about 6,000 and about 12,000.

It will be understood that other commonly employed additives such as corrosion inhibitors, oxidation inhibitors, stabilizers, metal deactivators, and the like, such as epoxides, dialkyl sulfides, benzothiazole, phenyl alpha naphthylamine and phenolic oxidation inhibitors, well known as functional fluid additives in the art, can also be incorporated in the functional fluid composition of the invention, in relatively small amounts.

The following are examples illustrating functional fluid compositions according to the invention.

6 EXAMPLES 14 Various compositions containing a phosphate ester and a dialkyl phthalate were prepared, and in some of these compositions a dialkyl adipate was also included, in accordance with the invention. In certain of these compositions a polyalkyl acrylate viscosity index improver was also added, and in some cases also an epoxide stabilizer and anti-corrosion and other conventional additives.

The density of the respective compositions and also the viscosity in centistokes at '40 F. and at 210 F., were measured.

The resulting compositions designated Examples 1 to 7 are given in the table below, together with their respective densities and viscosities at -40 F. and 210 F.

TABLE 1.EXAMPLES 0F COMPOSITIONS Components, percent by H s 7 9" 5 ON 17 07.0

From the table above, it is seen that the compositions containing a phosphate ester, specifically tributyl phosphate or tricresyl phosphate, and a dialkyl phthalate, specifically di isodec'yl phthalate, and in the absence of any other diester, as illustrated by the compositions of Examples 1, 6 and 7, each had densities ranging from 0.96 to 1.06, and are relatively low density fluids, and have viscosities at 40 F. ranging from 484 to 943 es, and at 210 F. ranging, from 2.41 to 4.6 cs. and within the requisite viscosities of less than 1,000 cs. at 40 F. and not more than 2.0 cs. at 210 F., so that such compositions are useful as functional or hydraulic fluids in aircraft.

It can also be seen from the compositions corresponding to Examples 2 to 5 of the table above, that upon the addition of a dialkyl adipate in the form of di isodecyl adipate, to the tributyl phosphate and di isodecyl phthalate components, generally the resulting fluid had somewhat lower density ranging from about 0.94 to about 0.96, the major effect of the addition of the di isodecyl adipate component being to substantially reduce the low temperature viscosity at -40 F. to a viscosity ranging from 327 to 672 cs.

It is noteworthy that in the compositions corresponding to Examples 1 to 4 and 7, which contain no polyalkyl acrylate viscosity index improver, both the low temperature viscosity at !-40 F. and the high temperature viscosity at 210 F. are within the above noted permissible parameters for viscosity required in hydraulic systems of aircraft. However, the addition of about 5% poly nhexyl acrylate to the compositions corresponding to Examples 5 and 6 did reduce low temperature viscosity at 40 F. to from 327 to 484 cs., while in Example 6 also increasing high temperature viscosity at 210 F. to 3.51 cs.

The compositions corresponding to Examples 1 to 7 of the above table also have good fire resistance and good thermal stability, for example, the compositions of Examples 3 and 4 have high autoignition temperatures of 870 F. and 880 F. respectively, as measured in accordance with ASTM D 2155 procedure.

7 EXAMPLES 8-15 TABLE 2.EXAMPLES OF COMPOSITIONS Components, percent by weight:

Tributyl phosphate 40 40 45 75 Dibutyl phenyl phosphate 60 30 20 Tricresyl phosphate 30 Phenyl dicresyl phosphate 20 Butyl diphenyl nbn nhate 20 Dibutyl phenyl phosphonate 40 Dipentyl phenyl phosph mime 30 Phenyl di-n-butyl phosphinate 55 Dibutyl phthalate 60 40 30 40 25 Dihexyl phthalate 40 45 Di(2ethylhexyl) adipate 30 lhexyl adipate Dioctyl sahacato Dllsodecyl sehacam From the foregoing, it is seen that in accordance with the invention, functional fluids designed for industrial and marine use, and particularly for use as hydraulic fluids in jet aircraft, are provided in the form of a mixture of a phosphorus ester, preferably a phosphate ester, and a phthalic acid diester, specifically a dialkyl phthalate, and which also can include an adipic acid diester and/or sebacic acid diester, in the form of a dialkyl adipate and/ or a dialkyl sebacate, which have requisite viscosity characteristics at temperatures of below -40 F. and above 210 F., permitting their use in aircraft hydraulic systems even in the absence of viscosity index improvers, and which have comparable fire resistance, thermal stability and corrosion and pump wear resistance of conventionally employed phosphate ester base stocks, and also have desirably low density, with the particular virtue that the resulting invention fluids in which a substantial proportion of phosphorus ester in conventionally employed phosphorus ester base stocks is replaced by phthalate diester, and also if desired by adipate and/or sebacate diesters, have substantially improved shear stability and substantially reduced cost as compared to the conventional phosphorus ester base stocks.

While I have described particular embodiments of my invention for purposes of illustration, it will be understood that various changes and modifications within the spirit of the invention can be made, and the invention is not to be taken as limited except by the scope of the appended claims.

I claim:

-1. A functional fluid composition which consists essentially of, by weight, (a) about 15 to about 75% of a phosphate ester in the form of a trialkyl phosphate containing alkyl groups of from about 3 to about 10 carbon atoms; (b) about 5 to about 75 of a phthalic acid alkyl diester containing alkyl groups of from about 4 to about 12 carbon atoms; and (c) 0 to about 50% of another diester compound selected from the group consisting of adipic and sebacic acid alkyl diesters, having alkyl groups containing from about 4 to about 12 carbon atoms.

2. A functional fluid composition as defined in claim 1, wherein said phosphate ester is present in amount ranging from about 20 to about 60%, said phthalic acid diester is present in an amount ranging from about 10 to about 50%, and said other diester compound is present in an amount ranging from about 10 to about 40%, by weight.

3. A functional fluid composition as defined in claim 1, consisting essentially of said phosphate ester and said phthalic acid diester.

4. A functional fluid composition as defined in claim 3, wherein said phosphate ester is present in amount ranging from about 40 to about and said phthalic acid diester is present in an amount ranging from about 15 to about 60%, by weight.

5. A functional fluid composition as defined in claim 1, consisting essentially of said phosphate ester, said phthalic acid diester, and said other diester compound.

6. A functional fluid composition as defined in claim 5, wherein said phosphate ester is present in amount ranging from about 20 to about 60%, said phthalic acid diester is present in an amount ranging from about 10 to about 50%, and said other diester compound is present in an amount ranging from about 10 to about 40%, by weight.

7. A functional fluid composition as defined in claim 1, wherein said phosphate ester is tributyl phosphate.

8. A functional fluid composition as defined in claim 7, wherein said phosphate ester is present in amount ranging from about 20 to about 60%, said phthalic aciddiester is present in an amount ranging from about 10 to about 50%, and said other diester compound is present in an amount ranging from about 10 to about 40%, by weight.

9. A functional fluid composition as defined in claim 7, wherein said phthalic acid diester is diisodecyl phthalate.

10. A functional fluid composition as defined in claim 2, wherein said phosphate ester is tributyl phosphate, and dibutyl phenyl phosphate, said phthalic acid diester is diisodecyl phthalate and said other diester compound is diisodecyl adipate.

11. A functional fluid composition as defined in claim 1, consisting essentially of from about 40 to about 75 tributyl phosphate and from about 15 to about 60% of diisodecyl phthalate, by weight.

12. A functional fluid composition as defined in claim 1, consisting essentially of about 15 to about 75% tributyl phosphate, about 5 to about 75% diisodecyl phthalate and an amount up to about 50% of diisodecyl adipate, by weight.

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Synthetic Lubricant Fluids From Branched-Chain Diesters, Industrial & Engineering Chemistry, vol. 39, pp. 484-497 (1947).

LEON D. ROSDOL, Primary Examiner A. PITLICK, Assistant Examiner US. Cl. X.R. 25249.8 

